1. Field of the Invention
The present invention relates to a method and apparatus for adjusting the focus in a camera and, more particularly, to a method and apparatus for adjusting the focus during scanning drive and non-scanning drive of a photographic lens.
2. Description of the Related Art
Conventional automatic focusing cameras typically use a charge accumulation type image, sensor to receive light of a subject image formed by a focus detection optical system. The image sensor output is analyzed and an amount of defocus of a photographic image plane with respect to a prearranged focus plane of a photographic optical system is calculated, based on the output of the image sensor. Optimum focus is achieved by driving a focusing lens in proportion to the calculated defocus amount.
In this type of conventional camera, a calculation of the defocus amount is impossible when the contrast of the photographic image is insufficiently low. Also, even if the contrast is high, the calculated defocus amount may be unreliable. Therefore, in situations where a calculation of the defocus amount is impossible, or where the calculated defocus amount is judged to be unreliable, the focusing lens is scanned between a close end position and an infinity end position to search for a position of the photographic optical system at which a reliable defocus amount can be calculated. This scanning of the focusing lens is called "scanning drive".
During scanning drive, there are a series of focus detection periods during which a charge is accumulated by the image sensor. A respective defocus amount is calculated for each focus detection period. During the focus detection periods, the amount of movement of the focusing lens of the photographic optical system is large. As a result, the focus position of the subject is missed during the scanning drive. This problem is illustrated in FIGS. 1(A) and 1(B), which are diagrams illustrating the position of a focusing lens of a photographic optical system during scanning drive. In FIGS. 1(A) and 1(B), points P1 through P6 represent positions of the focusing lens in the focus detection period. Moreover, the range W represents the range at which it is possible to detect a defocus amount in each focusing lens position, P1 through P6. The focus position Q represents the focus position of the focusing lens for the subject.
FIG. 1(A) illustrates a situation occurring in scanning drive in which the amount of movement of the focusing lens during the focus detection period is small in comparison with a detectable defocus amount. As illustrated in FIG. 1(A) the ranges W of the detectable defocus amount overlap in each focusing lens position P1, P2, P3 and P4. As a result, the focus position Q can be determined.
In contrast, FIG. 1(B) illustrates a situation occurring in scanning drive in which the amount of movement of the focusing lens during the focus detection period is large in comparison with a detectable defocus amount. As illustrated in FIG. 1(B), the amount of movement of the focusing lens in the focus detection period becomes larger than the detectable defocus amount and the ranges W of the detectable defocus amount do not overlap in each focusing lens position P5 and P6. As a result of a large movement of the focusing lens, it is impossible to determine the focus position Q.
A large movement of the focusing lens can be due to several factors. First, the charge accumulation time of the image sensor becomes long when the luminosity of the subject becomes low. Therefore, the focus detection period becomes long and the amount of movement of the focusing lens in the focus detection period becomes large. Second, if the scanning drive speed of the focusing lens is increased to reduce the scanning drive time, the amount of movement in the focus detection period becomes large. Third, in a multiple autofocus (multi-AF) camera, plural focus detection areas are disposed in the photographic picture plane. The camera performs focus detection for each focus detection area, plural defocus amounts are found and a final defocus amount is set from among these defocus amounts. The focusing lens is moved in accordance with the final defocus amount. Thus, the charge accumulation times, charge transfer times, focus detection calculation times and the time until the final defocus amount is calculated, become long. As a result, the focus detection period becomes long and the amount of movement of the focusing lens in the focus detection period becomes large.
Moreover, when the amount of movement of the focusing lens during the charge accumulation time of the image sensors becomes large, focus detection can be impossible because of movement in the contrast of the subject image. This movement of the contrast causes the contrast to decrease. As a result, the focus position Q cannot be determined.
Also, in a type of autofocus camera, light beams pass through different portions of the exit pupil position of a photographic optical system to form a pair of subject images. An image sensor receives light from the pair of subject images and a correlation calculation is performed to determine the relative shift of the pair of subject images. A focus adjustment state of a focusing lens is determined from the relative shift of the pair of subject images. However, the focus detection calculation time becomes long when the correlation calculation is performed during scanning drive at the same shift amount as in normal focus detection. Accordingly, as illustrated in FIG. I(B), the focus detection calculation time becomes long during scanning drive and the focus position cannot be obtained.
To reduce the focus detection calculation time, an autofocus camera has been proposed in which the focus detection calculation time is shortened by restricting the shift range of the, focus detection correlation calculation according to the direction of scanning of the focusing lens during scanning drive (see, for example, Japanese Laid-Open Patent Publication JP-A-63-172211). However, an adequate reduction of the focus detection calculation time is not sufficiently achieved by such a restriction of the shift range. Moreover, when taking photographs of moving subjects with the camera, a moving subject can move outside the focus detection region and thereby cause focus detection to be impossible. In this situation, focus detection is impossible when the shift range is restricted only in the scanning direction during scanning drive. Even if the subject returns to the focus detection region, focus detection may still be impossible.